The physical properties and processability of a rubber are key performance areas that the commercial rubber producer has to monitor. Amongst these are the level of unsaturation, the molecular weight distribution, the amount of ultraviolet stabilizer, the amount of antioxidant and the amount of plasticizer. Traditionally each of these parameters has been analyzed by an individual method and often this has proven to be difficult because the parameter is present at such a low concentration level.
The Iodine Index is presently accepted by the rubber industry as the method for determining the amount of unsaturation in an unsaturated rubbery copolymer such as a butyl rubber. However the method is not universally applicable to the quantitative determination of the level of unsaturation in a number of unsaturated rubbery copolymers as the amount of iodine that adds to each of the various isomeric olefinic structures in said unsaturated rubbery copolymers is not known. The method is further deficient in that it is operator dependent, it takes several hours to perform and other unsaturated species such as oligomers, ultraviolet stabilizer and antioxidant present in the sample of unsaturated rubbery copolymer submitted for analysis will, if not extracted, add to the final unsaturation value.
Analysis by nuclear magnetic resonance spectrometry of unsaturated rubbery copolymers in which the level of unsaturation is relatively high is possible. However nuclear magnetic resonance spectrometry is not capable of providing accurate unsaturation data on polymers with less than about three weight percent unsaturation as the instrument is not sufficiently sensitive. Moreover the cost of the instrument is such that the average rubber producer would not be prepared to install it in a quality control laboratory.
Hence there is a need in the industry for a method which is both rapid and accurate over a wide concentration range. Furthermore it would be advantageous if such a method could be placed in combination with at least one of the methods presently in use for the determination of one or more of the other parameters, as the cost of the analyses to the rubber producer would be reduced considerably.
To the best of Applicant's knowledge there is no report of an analytical method which is capable of measuring simultaneously the unsaturation level, the antioxidant level and the molecular weight distribution of an unsaturated elastomeric interpolymer selected from the group consisting of butyl rubbers and ethylene-propylene-nonconjugated nonconjugated diene terpolymers.
There are a number of patents which disclose the use of gel permeation chromatographs in combination with an appropriate detection system in the characterization of polymers. For example, U.S. Pat. No. 4,728,344 discloses the use of a gel permeation chromatograph in which the columns and, preferably the detector are heated, for determining the relative concentration and molecular weight of a component in a polymer which is soluble only above about 150.degree. C. However the Patent does not suggest using such a device to detect and quantify the unsaturation level in unsaturated elastomeric interpolymers nor does the Patent suggest that by placing a differential refractometer and a variable wavelength ultraviolet detector in series with the gel permeation chromatograph that it would be possible to measure simultaneously the unsaturation level, the antioxidant level and the molecular weight distribution of an unsaturated elastomeric interpolymer of one or more monoolefins and a diolefin.
Harrison, Yates and Johnson in the Journal of Liquid Chromatography, Volume 6, pages 2723-37 (1983) described the use of a gel permeation chromatograph in combination with a differential refractometer and an ultraviolet detector for a study of the aging of pressure-sensitive adhesive films based on isoprene-styrene block copolymers. The carbonyl chromophore formed on the room temperature oxidation of the thin films was monitored by the ultraviolet detector while the molecular weight distribution was measured with the differential refractometer. It was not suggested, however, that this system could be employed for the analysis of the molecular weight distribution and the amount of unsaturation in unsaturated elastomeric interpolymers of at least one monoolefin and a diolefin.
Grinshpun and Rudin described in Polymeric Materials in Science and Engineering, Volume 54 pages 174-179 (1986), the use of a gel permeation chromatograph in combination with three different detectors to study the molecular weight distributions of a series of ethylene-propylene-nonconjugated diene terpolymers having different amounts of unsaturation. The levels of unsaturation o the terpolymers, however, were to the best of Applicant's knowledge not determined using an ultraviolet detector in combination with the gel permeation chromatograph.
Kohn in U.S. Government Report DE86015001 describes methods for the identification and quantification of phenyl groups in molecular weight components of polydimethyl-siloxane prepolymers which utilize on-line differential refractometer, infrared and ultraviolet detection during gel permeation chromatographic separation. For the quantification of the phenyl groups it was found that a more polar solvent such as tetrahydrofuran or 1,4-dioxane which apparently complexes with the phenyl groups giving rise to enhanced absorbances that are less dependent on the position of the phenyl groups, is required.
Del Rios in paper No. 346 at the Pittsburgh Conference and Exposition, March 1986, focused on the use of a differential refractometer and an ultraviolet detector in series with a gel permeation chromatograph for determining the molecular weight distribution of a polymer together with the low molecular weight additives present in it. No reference was made to the possibility of analyzing the unsaturation content of unsaturated elastomeric interpolymers.